Luminescent screen



y 1941' J. H. DE BOER LUMINESCENT SCREEN Filed June 7, 1938 AAAA AAASOURCE OF VOL TA GE INVENTOR. JAN HENDR/K DE BOER ATTORNEY.

Patented May 20, 194i signer, ration of America, tion of Delaware bymesne assients,

to Radio Corpo- New York, N. Y., a corpora-v ApplicationJune 7, 1938,Serial No. 212,247 In Germany June I, 1937 9 Claims; (Ci. 250-2'Z.5)

This invention relates to luminescent screens and more particularly toan improved luminescent screen andthe method and means for preparingsuch screens for use in electron discharge devices wherein the screen isadapted to be impacted by electrons.

When the cathode rays falling on the fluorescent screen of a cathode raytube confer a negative charge on the screen, the tube, of course, cannotoperate correctly. For this reason it has been proposed to provide theluminous screen with a conductive layer to which is connected a sourceof potential for carrying ofithe charges of the electrons impinging onthe screen.

In the egqsence-of this conductive layer it is necessary at the screenshould emit at least as much secondary electrons as it collects primaryelectrons, otherwise the screen would be charged and the tube would notoperate or only in a defective manner. The necessary secondary emissioncan be attained when the electrons acquire a tial of the screen referredto the source of electrons will then become about equal to the anodepotential. necessary high velocity to produce, the secondary electrons,the potential difference through which the electrons pass must belikewise very high.

A cathode ray tube according to the invention comprises a fluorescentscreen whose power of -emitting secondary electrons is increased. For

this purpose a caesium oxide layer is adsorbed to the luminescentsubstance of the screen. By adsorbing caesium oxide to the luminescentmaterial, the emission of secondary electrons is 'so copious that thetube is adapted to operate at comparatively low voltages which, ofcourse, is an important advantage, as well as to overcome any distortionin light values which would arise from the charging of the screen.

With the method according to the invention, a caesium deposit is formedon the luminescent screen. The caesium adsorbed to the fluorescentsubstance is subsequently oxidized by means of oxygen.

It has been found that a screen made accord ing to this method has sucha capacity of emitting secondary electrons that the desired equi libriumis obtained and charging of the screen is avoided even at an electronspeed corresponding to a voltage comprised between 200 and 300 volts.The method according to the invention However, to give the electrons thesufficiently high velocity. The potenmay be carried out in a very simplemanner by providing that the caesium and the oxygen used for oxidizingit are set free from a capsule inagent satisfying side the tube itself.The free caesium may either be provided in a hermetically closedenvelope or a mixture is used containing a caesium compound whichreadily evolves caesium by reduction. Such method and means forreleasing caesium are well known in the prior art and are shown inPatents Nos. 1,738,420, entitled Method of preparing electric dischargedevice. issued December 3, 1929, to Charlton, 1,835,117, entitledIntroduction of alkali metals into evacuated containers, and 1,835,118;entitled Alkali metal tubes issued on December 8, 1931, to Marden. Forthis purpose a mixture of caesium chromate and zirconium may bypreference be used. I For releasing oxygen 2. capsule is preferably usedcontaining acompound which by heating to a temperature exceeding thepumping temperature of the tube evolves oxygen. A suitable thiscondition is MnOz which does not release oxygen to any considerableextent below'600 to 700 C. In this case the tube can normally beexhausted after providing the material for the luminescent screen andassembling the bulb and the electrode system.

The luminescent material may, for instance, be an activated zincsilicate which is provided on the surface of the screen by means of I abinder such as nitro-cellulose. The binder isflsubsequently burnt awayand the zinc silicate is gently sintered. After that; caesium vapour isintroduced in the vacuum space. The tube is preferably heated to such a'temperature that the caesium does not form a permanent deposit on theparts of the tube, whilst the excess caesium vapour is removedbypumping. The caesium is left as an adsorbed layer only on theluminescent screen due 0 which the latter acquires a brown color. Withfurther high temperature treatment and sintering, it displays ablue-color. It is understood, of-course, that other methods ofdepositing the luminescent material upon the supporting surface, suchasthe end wall of, the cathode ray tube, may be used. Such methods includeeither spraying the luminescent material onto the surface, or bysettling out the-material from a suspension onto the end wall.

Subsequently oxygen is introduced into the vacuum space and the caesium,which has a high aflinity to oxygen, is completely oxidized due to whichthe screen acquires again its initial white color.

Preferably, the cathode is activated in a manner known per se only afterremoving the free oxygen, in order that it shall not beattacked by theoxygen used for oxidizing the caesium.

In the drawing I have shown schematically a cathode ray tube embodying aluminescent screen upon which is absorbed caesium oxide. The cathode raytube I is provided with a cathode 3, a concentrating or controlelectrode 5, a first anode I, and a second anode 9 formed by aconductive layer deposited within and on the tube wall.

constitute the means for producing a focused beam of electrons which isto be projected upon the luminescent screen l3 deposited upon the endwall ll of the tube I. The luminescent screen l3 deposited upon the wallis formed from any suitable fluorescent material, such as an activatedzinc silicate, upon which material there is absorbed a layer of caesigen bearing compound, which is to be decomposed.

In practice, the tip l5 frame 25 until the chemical reaction takesplace,

the caesium layer by application of heat to the tip 2| to produce thedecomposition of the oxygen bearing compound, the tip 2| is sealed 011'.

Having described my invention, what I claim is:

1. The method of preparing an improved luminescent screen whichcomprises the steps 01' depositing a luminescent supporting surface,thermally decomposing a posited material, heating the condensed caesiumend would be heated by a to produce a predetermined color, thermallydecomposing manganese oxide to produce oxygen, and oxidizing the heatedcondensed caesium by the produced oxygen.

2. The method of preparing an improved luminescent screen whichcomprises the steps 01' depositing a luminescent material upon asupporting surface, thermally decomposing a caesium compound to producecaesium vapor, condensing the produced vapor upon the depositedmaterial, heat treating the condensed caesium to produce a blue color,and subsequently oxidizing the heat treated caesium.

3. The method of preparing an improved luminescent screen whichcomprises the steps of depositing a luminescent material upon asupporting surface, thermally decomposing a caesium compound to producecaesium vapor, condensing the produced vapor upon the depositedmaterial, sintering the condensed caesium to produce a blue color, andsubsequently oxidizing the sintered caesium.

upon a supporting surface, producing caesium vapor, adsorbing theproduced vapor to the brown color to the adsorbed caesium.

5. The method of preparing an improved luminescent screen whichcomprises the steps 01 color to blue, thermally decomposing an oxygenbearing compound to produce oxygen, and oxidizing the heated adsorbedcaesium by the produced oxygen.

6. The method of preparing an improved luminescent screen whichcomprises the steps of depositing a luminescent-material upon asupporting surface, reducing a caesium compound to produce caesiumvapor, condensing the produced vapor upon the deposited material toimpart a brown color thereto, heating the condensed caesium to altersaid brown color-to blue, thermally decomposing an oxygen bearingcompound to produce oxygen, and oxidizing the heated adsorbed caesium bythe produced oxygen.

7. The method of preparing a secondary electron emissive luminescenttarget which comprises depositing luminescent material 'upon a supportmember, producing a caesium vapor, condensing the produced caesium vaporand forming an adsorbed caesium coating upon the deposited luminescentmaterial, decomposing an oxide compound to produce oxygen and oxidizingthe condensed adsorbed caesium by the produced oxygen.

8. The method of preparing a secondary electron emissive luminescenttarget which comlease secondary electrons under electron beam impact.

- JAN HENDRIK n1: BOER.

